Resonant network with reactively coupled fet providing linear voltage/frequency response

ABSTRACT

A field-effect transistor with two gates has a resistance and a reactance serially connected across its drain and source, with one gate tied to the junction of these two impedances whereas the other gate has a modulating signal applied to it. The resistance may be constituted by the output circuit of a differential transistor amplifier having its input connected in parallel with drain and source of the field-effect transistor.

Inventor Rudoli Dick Enlngen, Germany Appl. No. 880,116 Filed Nov. 26,1969 Patented Nov. 16, 1971 Assignee Wandel u. Goltermann Reutllngen,Germany Priority Nov. 27, 1968 Germany RESONANT NETWORK WITH REACTIVELYCOUPLED FET PROVIDING LINEAR VOLTAGE/FREQUENCY RESPONSE [56] RellerencesCited UNITED STATES PATENTS 2,382,436 8/1945 Marble 332/28 UX 2,441,5045/1948 332/28 X 2,521,694 9/1950 332/28 X 2,749,518 6/1956 332/282,758,211 8/1956 Hochman 332/28 X 3,436,681 4/1969 Hart 331/117 OTHERREFERENCES Butler, Application of Metal Oxide Silicon Transistors,Wireless World, Feb. 1965, pp. 58- 61 .33 l 108 Primary Examiner-AlfredL. Brody Aitorney- Karl F. Ross ABSTRACT: A field-effect transistor withtwo gates has a regchimssnmwing Figs sistance and a reactance seriallyconnected across its drain 11.8. C1 332/16 T, and source, with one gatetied to the junction of these two im- 307/251, 331/108, 332/28, 333/80Tpedances whereas the other gate has a modulating signal ap- Int.Cl 1103c3/10 plied to it. The resistance may be constituted by the output Fieldof Search 332/16, 16 circuit of a differential transistor amplifierhaving its input T, 28; 307/25], 304; 331/108, 117, 177, 177 V,connected in parallel with drain and source of the field-effect 180;333/80, 80 T transistor.

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FIG.

Rudolf Dick INVliN'IUR ss 9'63!) Attorney RESONANT NETWORK WITHREACTIVELY COUPLED FET PROVIDING LINEAR VOLTAGE/FREQUENCY RESPONSE Mypresent invention relates to an impedance network constituting orincluding an electrically adjustable reactance, e.g., for use in thetank circuit of a tunable oscillator In the tuning of resonant circuitsfor oscillators and the like it is desirable to have a substantiallylinear relationship between the resonance frequency and the controlvoltage applied to the reactive branch of the circuit. In conventionalsystems in which this reactive branch includes an active element of thevariable-gain type, such linearity is generally not realizable withoutthe use of separate equalizing networks or the like. These equalizersare, as a rule, of complex construction even when designed for thelinearization of only a relatively narrow frequency range.

It is, therefore, the principal object of my present invention toprovide an improved and uncomplicated impedance network whose reactanceZ varies generally according to the law Z=KIV where V is the appliedvoltage. This relationship provides the desired linearity, in regard tothe resonance frequency f, according to the well-known formula where theinductance L or the capacitance C is constituted by the adjustablereactance Z.

A more specific object is to provide an impedance network with aninductive or capacitive reactance variable in the aforedescribed mannerover an extended range.

These objectsare realized, in accordance with my inven tion, by the useof a solid-state active element with two output electrodes and twocontrol electrodes, one control electrode being tied to the junction ofa predominantly reactive first impedance and a preferably resistivesecond impedance serially connected across a pair of load terminals as aphase-shifting circuit; upon the application of a modulating voltage tothe other control electrode, the effective reactance measured acrossthese terminals varies as a function of the applied voltage. Thisreactance will be either capacitive or inductive, depending on thecharacter and position of the predominantly reactive series resistanceof the phase shifter.

l have found pursuant to a more particular feature of this invention,that an especially good linearization of the resulting resonancefrequency (in an oscillatory circuit in which this network forms one ofthe reactive branches) is achieved by the use of a twin-gatefield-effect transistor as the solid-state active element.

The second, predominantly resistive series impedance of the phaseshifter bridged across the load terminals may be constituted by anancillary amplifier having its input circuit connected across theseterminals. Advantageously, this ancillary amplifier comprises twodifferentially connected transistor stages, the first transistor stagehaving its base/collector circuit in parallel with the source/draincircuit of the field-effect transistor so as to be responsive to thereactive voltage developed across the latter transistor; theemitter/collector circuit of the second stage then constitutes ahighohmic branch of the phase-shifting circuit.

The invention will be described in greater detail hereinafter withreference to the accompanying drawing in which:

FIG. 1 is a circuit diagram of an impedance network embodying theinvention;

FIG. 2 is a diagram similar to FIG. 1, showing a modification; and

FIG. 3 is a circuit diagram of a more elaborate impedance networkgenerally similar to that of FIG. 1.

The embodiment illustrated in FIG. 1 comprises a pair of load terminals1, 2, terminal 2 being grounded; an alternating voltage of pulsatance wfrom an external source (not shown) is assumed to be present on theseterminals. A twin-gate field-effect transistor 3 has its drain andsource electrodes respectively connected to these terminals in parallelwith a phase-shifting circuit consisting of a capacitor 41 and aresistor 5. One gate of field-effect transistor 3 is tied to thejunction of impedances 4 and 5; the other gate is connected to aterminal 6 to receive a modulating voltage from a source V lying betweenthis terminal and a grounded terminal 8. Source V is representative ofany manually or automatically adjustable means for producing a controlvoltage of variable magnitude; the polarity of that voltage depends, ofcourse, on the conductivity type of the field-effect transistor 3.

With element 5 designed as a high-ohmic resistance, the impedance of thetwo-terminal network 3 5 as seen from terminals 1. 2 is essentiallyinductive and. given by jwL, L varying substantially linearly with thereciprocal of the square of the applied control voltage V,,, Thus, thenetwork can be used as a reactive branch of an oscillatory circuithaving a complementary (here capacitive) reactive branch in parallel orin series therewith, as diagrammatically illustrated by a condenser 30in FIG. ll.

FIG. 2 shows a similar network of inductive character wherein, however,the phase-shifting circuit controlling the field-effect transistor 3 isconstituted by an inductance 7 in series with a resistor 9. If theinductance 7 were replaced by the capacitance 4 of FIG. 1, or viceversa, the character of the network impedance would be capacitive.

In FIG. 3 I have shown a network similar to that of FIG. I wherein,however, the resistive branch 5 of the phase shifter has been replacedby a two-stage amplifier 15 comprising a pair of PNP-transistors 10 and11. The emitter and collector of the first transistor stage 11 areconnected in series with a resistor 13 between ground and a bus bar 23leading to a source of positive biasing potential +U in parallel with avoltage divider constituted by two series resistors 20, 21 whosejunction is tied to the base of transistor 11; this junction is alsoconnected through a coupling condenser 17 to the ungrounded loadterminal 1. The base of the second transistor stage 10 is grounded forhigh frequency through another coupling condenser 16 and is further tiedto the junction of two series resistors 18, 19 inserted between bus bar23 and ground. Bus bar 23 is also connected to the emitter of transistor10 through a resistor 12 forming part of a IFHGIWOIK which includes theemitter resistor 13 of transistor 11 as well as a further resistor 14.The collector of transistor 10, aside from being connected to thejunction of condenser 4 with the first gate of field-effect transistor3, is led to a source of negative biasing potential U through ahigh-ohmic resistor 22.

Amplifier 15, lying in a feedback path of transistor 3, magnifies theeffect of the modulating voltage V,,, upon the reactance jmL oftransistor 3; the high base/collector resistance of amplifier stage 11and the high-ohmic resistors 20 and 21 have only a negligible shuntingeffect upon this reactance. The principle of combining a feedbackamplifier with a controlled reactance element is known per se, e.g. fromGerman printed specification No. 1,274,679 to which reference may bemade for a determination of the mathematical relationships between theseveral impedances of the network.

The field-effect transistor 3 is advantageously of themetaloxide-semiconductor type, known as MOSFET, described for example inthe June 1967 issue oflEEE spectrum pp. 50 58.

I claim:

1. A two-terminal impedance network comprising a pair of load terminalsenergizable by alternating current; a field-ef fect transistor with asource and a drain respectively connected to said terminals, saidtransistor having a first and a second gate electrode; a pair ofimpedances serially connected across said terminals, at least one ofsaid impedances being predominantly reactive, said first gate electrodebeing connected to the junction of said impedances; and a supply ofmodulating signal connected between said second gate electrode and oneof said terminals.

2. A network as defined in claim 1 wherein said field-effect transistoris of the metal-oxide-semiconductor type.

3. A network as defined in claim ll wherein one of said impedances ispredominantly resistive.

4. A network as defined in claim 3 wherein said predominantly resistiveimpedance comprises an output circuit of an ancillary amplifier havingan input circuit connected across said terminals.

5. A network as defined in claim 4 wherein said ancillary am lifiercomprises two differentially connected transistor stages.

6. A network as defined in claim 5 wherein said stages each have a base,an emitter and a collector, said source and drain being connected acrossthe base/collector circuit of one of said transistor stages, the otherof said transistor stages having an emitter/collector circuit in serieswith said predominantly reactive impedance.

1. A two-terminal impedance network comprising a pair of load terminalsenergizable by alternating current; a field-effect transistor with asource and a drain respectively connected to said terminals, saidtransistor having a first and a second gate electrode; a pair ofimpedances serially connected across said terminals, at least one ofsaid impedances being predominantly reactive, said first gate electrodebeing connected to the junction of said impedances; and a supply ofmodulating signal connected between said second gate electrode and oneof said terminals.
 2. A network as defined in claim 1 wherein saidfield-effect transistor is of the metal-oxide-semiconductor type.
 3. Anetwork as defined in claim 1 wherein one of said impedances ispredominantly resistive.
 4. A network as defined in claim 3 wherein saidpredominantly resistive impedance comprises an output circuit of anancillary amplifier having an input circuit connected across saidterminals.
 5. A network as defined in claim 4 wherein said ancillaryamplifier comprises two differentially connected transistor stages.
 6. Anetwork as defined in claim 5 wherein said stages each have a base, anemitter and a collector, said source and drain being connected acrossthe base/collector circuit of one of said transistor stages, the otherof said transistor stages having an emitter/collector circuit in serieswith said predominantly reactive impedance.
 7. A circuit arrangementincluding a resonant circuit with a first reactive branch comprising anetwork as defined in claim 1 and a second, complementary reactivebranch connected to said load terminals.
 8. A circuit arrangement asdefined in claim 7 wherein said predominantly reactive impedance isconstructed and positioned to make the impedance of said networkessentially inductive.
 9. A circuit arrangement as defined in claim 8wherein said second reactive branch comprises a condenser connectedacross said load terminals.